The proposed in vitro study will be addressed to leukocyte/material interaction under various flow conditions characterized by different shear stresses and flow durations. The flow conditions will be controlled by concentric rotating cylinders which will be coated by several potentially useful biomaterials such as Biolite, Avcothane, Biomer, and siliconized stainless steel. The complexity of leukocyte alterations in response to the combined effects of material surface and fluid dynamic shear will be studied first when red blood cells and platelets are removed. The threshold values of shear and duration on leukocyte/material interaction will be measured for human and calf blood. The comparison of these two leukocyte behaviors, although in vitro, will be useful to development of calf models commonly employed in cardiopulmonary flow studies, prostheses development, and their clinical and surgical applications. This data will be compared with the response of leukocytes in whole human and calf blood after exposure to artificial surfaces in well controlled shear fields.